Those are not the words of a Dark Lord who doesn't care about your opinion and is about to wipe your memory.

They are precisely the words of a mysterious person who's trying to persuade you of something. It's simple flattery.

Eliezer thoroughly deconstructed Dumbledore's (And Gandalf's) view of evil in Lord of the Rationality.

Excellent point, but I hardly think this is a Sauron-level mistake. He may not absolutely fail to consider the actions and thoughts of moral people, but that doesn't mean it doesn't take him a few tries to find what buttons to push on an almost absolutely moral little girl.

Quirrell has to have some weakness, after all, if Harry is to ever beat him, as he presumably will. Why couldn't it be that Quirrell is truly cynical and does truly believe all people act selfishly most of the time? Why couldn't that omake be a foreshadowing of Quirrell's downfall? Quirrell would never have left Mount Doom unguarded, but that doesn't mean he won't make some other, smaller critical mistake.

He might have revealed a weakness to Harry, even as brilliant as he is. He certainly doesn't love Harry, he's certainly not fond of him, but I think he feels a kinship with him, given that Harry houses a piece of Voldemort's mind/soul/whatever. In his effort to turn Harry Dark, he might yet reveal more than he should.

Quirrell doesn't have to be perfect. If he were, then Harry could never defeat him. Just that he never holds the Idiot Ball doesn't mean he doesn't make small, insignificant-seeming mistakes that may haunt him later.